How can I evaluate the performance of a system that generates text?
This is a tricky issue. I assume you are using transition probabilities to pick the next suitable word, so you could use some other corpus data, derive probabilities from it, and compare  those to your system. Not very satisfactory, though, as you might end up evaluating the system in a circular way, deriving your test data in the same way as you generate your output.

A better, if somewhat more laborious evaluation would be to present the trigger sequence to human beings (on a fairly large scale), and then measure the overlap between your system and the human choices. Of course, with the you will get a very large set of possible replies, but the round yellow would be a lot more restricted (and it would make more sense to include that kind of sequence in an evaluation anyway).

The figure you will get might be quite small (depending on the range of possible options), but that is to be expected, so your system could score well even with a small overlap only. That's how I would approach this problem.